Stand-alone photovoltaic (PV) system is considered as promising renewable power sources for remote areas, independent home unit and street light. A typical model of this system, as shown by the block diagram in FIG. 1, was composed by the PV arrays the energy convert unit (101), storage batteries (103) and the system control unit (102). It converts the solar energy into electrical energy, stores it into the battery system during the day and then re-loads it during evening or when required. The charge and discharge of battery is controlled by the control unit. In real application, the energy storage system (here are the batteries) needs to have sufficient capacity for ensuring the stored energy which can be last a few days for continuous cloudy days. In this case, the energy storage system could not be fully charged during the day, but needs discharged during the night. The energy storage system batteries in a stand-alone PV system normally has been cycled between 20˜80% deep of discharge (DoD), i.e., under Partial State of Charge (PSoC).
Maximum power point tracking (MPPT) is widely published technology in recent years for increasing the charge efficiency of PV system. However, the energy storage system (the inherent character of the storage battery) normally has a high charging acceptance at a lower State of Charge (SOC) and a lower charging acceptance at a high SOC. Stand-alone PV system normally employs valve regulated lead acid (VRLA) battery for its energy storage. In order to enable fast and fully charge the batteries in the energy storage system, people prefers to install a PV array with high power output, but a VRLA battery, the gassing point is normally around 2.35V/per cell under the temperature of 25° C., depends on the environment, the electrolyte density and SOC. Once reaching the gassing point, the charging acceptance of the battery will be reduced, part of the input energy (the charging current) goes to the side reaction: electrolysis-gassing. Once reach the gassing point, the higher charging current and the higher gassing. The high gassing not only wastes the input energy, worse on that, it accelerates the battery aging and reduces the battery service life.
Moreover, one most concerned is that the storage batteries (the group of battery) of the stand-alone PV system, under the current technology and current service condition, often operated under the PSoC condition, as continuous cloudy days will make the storage batteries in sufficiently charged, if frequently repeated and prolonged, severe negative plate non-reversible sulfation in the lead acid cells will happen, even with enough charging power from PV system by following shining days. This will result the battery capacity lose and reduced service life.
This will happen to all kinds of PV systems, including the stand-alone PV system and the Wind-PV Hybrid system. Because the instability of the original energy, either from solar or wind.
It has been reported by Wu Bo in the journal of Electrical and Electronic Technologies (Chinese, February 2008) that “The design and application of group of batteries for PV system”, has described a method to charge the lithium battery by alternatively with constant current and constant voltage.
The Chinese patent No. CN02282038.8 disclosed “a pulse charging method for PV system” with a pulse charging to replace the constant floating charging after the rechargeable battery has been fully charged.
For improving the cycle life of battery, the present applicant in his Chinese patent No. CN01117645.5 disclosed a unique charging method with pulse charging/discharging, and with measured pulse current, the voltage of gassing point, temperature compensation to predict the SOC. Those are enclosed as the background of the invention.